This invention relates to fatty acid desaturases and nucleic acids encoding desaturase proteins. More particularly, the invention relates to nucleic acids encoding delta-12 and delta-15 fatty acid desaturase proteins that affect fatty acid composition in plants, polypeptides produced from such nucleic acids and plants expressing such nucleic acids.
Many breeding studies have been conducted to improve the fatty acid profile of Brassica varieties. Pleines and Freidt, Fat Sci. Technol., 90(5), 167-171 (1988) describe plant lines with reduced C18:3 levels (2.5-5.8%) combined with high oleic content (73-79%). Rakow and McGregor, J. Amer. Oil Chem. Soc., 50, 400-403 (October 1973) discuss problems associated with selecting mutants for linoleic and linolenic acids. In. Can. J. Plant Sci., 68, 509-511 (April 1988) Stellar summer rape producing seed oil with 3% linolenic acid and 28% linoleic acid is disclosed. Roy and Tarr, Z. Pflanzenzuchtg, 95(3), 201-209 (1985) teaches transfer of genes through an interspecific cross from Brassica juncea into Brassica napus resulting in a reconstituted line combining high linoleic with low linolenic acid content. Roy and Tarr, Plant Breeding, 98, 89-96 (1987) discuss prospects for development of B. napus L. having improved linolenic and linolenic acid content. European Patent application 323,753 published Jul. 12, 1989 discloses seeds and oils having greater than 79% oleic acid combined with less than 3.5% linolenic acid. Canvin, Can. J. Botany, 43, 63-69 (1965) discusses the effect of temperature on the fatty acid composition of oils from several seed crops including rapeseed.
Mutations typically are induced with extremely high doses of radiation and/or chemical mutagens (Gaul, H. Radiation Botany (1964) 4:155-232). High dose levels which exceed LD50, and typically reach LD90, led to maximum achievable mutation rates. In mutation breeding of Brassica varieties high levels of chemical mutagens alone or combined with radiation have induced a limited number of fatty acid mutations (Rakow, G. Z. Pflanzenzuchtg (1973) 69:62-82). The low xcex1-linolenic acid mutation derived from the Rakow mutation breeding program did not have direct commercial application because of low seed yield. The first commercial cultivar using the low xcex1-linolenic acid mutation derived in 1973 was released in 1988 as the variety Stellar (Scarth, R. et al., Can. J. Plant Sci. (1988) 68:509-511). Stellar was 20% lower yielding than commercial cultivars at the time of its release.
Alterations in fatty acid composition of vegetable oils is desirable for meeting specific food and industrial uses. For example, Brassica varieties with increased monounsaturate levels (oleic acid) in the seed oil, and products derived from such oil, would improve lipid nutrition. Canola lines which are low in polyunsaturated fatty acids and high in oleic acid tend to have higher oxidative stability, which is a useful trait for the retail food industry.
Delta-12 fatty acid desaturase (also known as oleic desaturase) is involved in the enzymatic conversion of oleic acid to linoleic acid. Delta-15 fatty acid desaturase (also known as linoleic acid desaturase) is involved in the enzymatic conversion of linoleic acid to xcex1-linolenic acid. A microsomal delta-12 desaturase has been cloned and characterized using T-DNA tagging. Okuley, et al., Plant Cell 6:147-158 (1994). The nucleotide sequences of higher plant genes encoding microsomal delta-12 fatty acid desaturase are described in Lightner et al., WO94/11516. Sequences of higher plant genes encoding microsomal and plastid delta-15 fatty acid desaturases are disclosed in Yadav, N., et al., Plant Physiol., 103:467-476 (1993), WO 93/11245 and Arondel, V. et al., Science, 258:1353-1355 (1992). However, there are no teachings that disclose mutations in delta-12 or delta-15 fatty acid desaturase coding sequences from plants. There is a need in the art for more efficient methods to develop plant lines that contain delta-12 or delta-15 fatty acid desaturase gene sequence mutations effective for altering the fatty acid composition of seeds.
The invention comprises Brassicaceae or Helianthus seeds, plants and plant lines having at least one mutation that controls the levels of unsaturated fatty acids in plants. One embodiment of the invention is an isolated nucleic acid fragment comprising a nucleotide sequence encoding a mutation from a mutant delta-12 fatty acid desaturase conferring altered fatty composition in seeds when the fragment is present in a plant. A preferred sequence comprises a mutant sequence as shown in FIG. 2. Another embodiment of the invention is an isolated nucleic acid fragment comprising a nucleotide sequence encoding a mutation from a mutant delta-15 fatty acid desaturase. A plant in this embodiment may be soybean, oilseed Brassica species, sunflower, castor bean or corn. The mutant sequence may be derived from, for example, a Brassica napus, Brassica rapa, Brassica juncea or Helianthus delta-12 or delta-15 desaturase gene.
Another embodiment of the invention involves a method of producing a Brassicaceae or Helianthus plant line comprising the steps of: (a) inducing mutagenesis in cells of a starting variety of a Brassicaceae or Helianthus species; (b) obtaining progeny plants from the mutagenized cells; (c) identifying progeny plants that contain a mutation in a delta-12 or delta-15 fatty acid desaturase gene; and (d) producing a plant line by selfing or crossing. The resulting plant line may be subjected to mutagenesis in order to obtain a line having both a delta-12 desaturase mutation and a delta-15 desaturase mutation.
Yet another embodiment of the invention involves a method of producing plant lines containing altered fatty acid composition comprising: (a) crossing a first plant with a second plant having a mutant delta-12 or delta-15 fatty acid desaturase; (b) obtaining seeds from the cross of step (a); (c) growing fertile plants from such seeds; (d) obtaining progeny seed from the plants of step (c); and (e) identifying those seeds among the progeny that have altered fatty acid composition. Suitable plants are soybean, rapeseed, sunflower, safflower, castor bean and corn. Preferred plants are rapeseed and sunflower.
The invention is also embodied in vegetable oil obtained from plants disclosed herein, which vegetable oil has an altered fatty acid composition.
SEQ ID NO:1 shows a hypothetical DNA sequence of a Brassica Fad2 gene. SEQ ID NO:2 is the deduced amino acid sequence of SEQ ID NO:1.
SEQ ID NO:3 shows a hypothetical DNA sequence of a Brassica Fad2 gene having a mutation at nucleotide 316. SEQ ID NO:4 is the deduced amino acid sequence of SEQ ID NO:3.
SEQ ID NO:5 shows a hypothetical DNA sequence of a Brassica Fad2 gene. SEQ ID NO:6 is the deduced amino acid sequence of SEQ ID NO:5.
SEQ ID NO:7 shows a hypothetical DNA sequence of a Brassica Fad2 gene having a mutation at nucleotide 515. SEQ ID NO:8 is the deduced amino acid sequence of SEQ ID NO:7.
SEQ ID NO:9 shows the DNA sequence for the coding region of a wild type Brassica Fad2-D gene. SEQ ID NO:10 is the deduced amino acid sequence for SEQ ID NO:9.
SEQ ID NO:11 shows the DNA sequence for the coding region of the IMC 129 mutant Brassica Fad2-D gene. SEQ ID NO:12 is the deduced amino acid sequence for SEQ ID NO:11.
SEQ ID NO:13 shows the DNA sequence for the coding region of a wild type Brassica Fad2-F gene. SEQ ID NO:14 is the deduced amino acid sequence for SEQ ID NO:13.
SEQ ID NO:15 shows the DNA sequence for the coding region of the Q508 mutant Brassica Fad2-F gene. SEQ ID NO:16 is the deduced amino acid sequence for SEQ ID NO:15.
SEQ ID NO:17 shows the DNA sequence for the coding region of the Q4275 mutant Brassica Fad2-F gene. SEQ ID NO:18 is the deduced amino acid sequence for SEQ ID NO:17.